U.S. Pat. No. 4,414,316, issued to K. E. Conley on Nov. 8, 1983, describes several prior methods of making relief patterned sheets for such applications as relatively thin, flexible Fresnel lenses and for projection screens. One such method includes stamping or embossing by a platen press. This method involves placing a thermoplastic material in a platen press having an engraved surface corresponding to the desired relief pattern. The platen press is closed and subjected to a heating cycle, to heat and soften the thermoplastic material and cause it to conform to the engraved surface of the platen press. The platen press then undergoes a cooling cycle to cool and harden the thermoplastic material, so that it will retain its shape and can be removed from the press. Because of the required heating and cooling cycles, the platen press method is slow and relatively expensive. Also, the platen press has a tendency to entrap bubbles. Other methods are more suited to producing relief patterns in continuous length form. Typically, such continuous methods utilize an engraved roll, and the relief pattern is formed by the roll embossing the thermoplastic material while in a softened or molten state. However, such a process cannot be used to manufacture a Fresnel lens, since such a lens has a sawtooth structure that- would.. interlock with the pattern on the roll. Because of the elastic nature of the thermoplastic material and the internal stresses imparted by the embossing roll, the embossed sheet produced by this method has a tendency to return to its original flattened and unstressed configuration. Therefore, the particular lens-like or other configurations which are formed in the sheet change in shape from their originally desired shapes, thus losing some of their desired optical qualities. Also, sheets produced by this method are highly susceptible to streaking during the manufacturing operation, as a result of condensation on the sheet during cooling. Additionally, the sheet is susceptible to dimensional distortion by the embossing machinery unless the roll speed and web tension are accurately maintained.
These problems are overcome by the use of thermosetting polymers, rather than thermoplastics. Cured thermosetting polymers are not subject to distortion or change in dimension with heat or moisture content, nor do they exhibit a tendency to change shape resulting from elastic memory imparted in producing the relief pattern. The preferred thermosetting materials are those that can be cured by actinic radiation, such as UV or electron beam radiation, in addition to heat curing. The above-referenced U.S. Pat. No. 4,414,316 discloses a method of making a composite sheet comprising a base film which is coated with a flowable uncured thermosetting resin with a molding surface having the desired lenticular pattern formed therein. To enhance the adhesion between the base film and the thermosetting resin layer, a primer or adhesion promoter preferably is applied to the front surface of the film, before the application of the resin. The resin is then cured, either thermally or by actinic radiation, to cause it to harden. The relief patterned thermosetting polymer layer is adherently and permanently bonded to the base film. The composite sheet material is utilized either in this form or it may have additional coating layers or laminations applied thereto. The thickness of the composite sheet ranges from about 0.15 to 0.76 mm. For other end use applications, the composite sheet material may be used as an intermediate product in producing sheet materials with predetermined relief patterns formed in both surfaces thereof. For such applications, the relief patterned thermosetting polymer layer is bonded to the base film with a relatively weak bond strength which permits stripping of the base film from the thermosetting polymer layer for further subsequent processing of the thermosetting polymer layer. However, the resultant structure is described as relatively thin and flexible, and therefore not suitable for use in a projection screen, without bonding the composite sheet to a suitable backing plate.
U.S Pat. No. 5,066,099, issued to T. on Nov. 19, 1991, describes a rear projection screen and a method of producing the same. In the patented structure, a UV curable material is flow coated onto the lenticular emergence surface of the screen, to form a light diffuser having a higher hardness than that of the underlying sheet material. There is no suggestion in the patent that UV curable materials can be patterned to make any of the lens components of the screen.
A need thus exists for a simple structure and method wherein UV curable materials may be used to form lens elements on substantially rigid transparent materials, without lamination or the use of additional primers or adhesion promoters.